Electric power cables for low voltages, i.e. voltages of below 6 kV, usually comprise an electric conductor which is coated with an insulation layer. Such a cable will in the following be referred to as single wire cable. Optionally, two or more of such single wire cables are surrounded by a common outermost sheath layer, the jacket.
The insulation layer of low voltage power cables usually is made of a polymer composition comprising a polymer base resin, such as a polyolefin. A material commonly used as a base resin is polyethylene.
Furthermore, in the final cable the polymer base resin usually is cross-linked.
In addition to the polymer base resin, polymer compositions for insulation layers of low voltage power cables usually contain further additives to improve the physical properties of the insulating layer of the electric cable and to increase its resistance to the influence of different surrounding conditions. The total amount of the additives is generally about 0.3 to 5% by weight, preferably about 1 to 4% by weight of the total polymer composition. The additives include stabilizing additives such as antioxidants to counteract decomposition due to oxidation, radiation, etc.; lubricating additives, such as stearic acid; and cross-linking additives such as peroxides to aid in the cross-linking of the ethylene polymer of the insulating composition.
In contrast to low voltage (<6 kV) power cables, medium (>6 to 68 kV) and high voltage (>68 kV) power cables are composed of a plurality of polymer layers extruded around an electric conductor. The electric conductor is coated first with an inner semiconductor layer followed by an insulating layer, and then an outer semiconductive layer all based on crosslinked polyethylene. Outside this cable core layers consisting of water barriers, metallic screens, bedding (polymer layer making the cable round) and on the outside a polyolefin based sheath layer are commonly applied. The thickness of the insulation layer of these cables is in the range of 5 to 25 mm.
As in low voltage power cables the insulation layer is usually much thinner, e.g. 0.4 to 3 mm, and directly coated onto the electric conductor and the insulation layer being the only layer surrounding each single conducting core, it is of great importance that the insulation layer must have good mechanical properties, like elongation at break and tensile strength at break. However, when this thin polyolefin layer is extruded towards a cold conductor, its mechanical properties are heavily deteriorated. For this reason, when extruding insulation layers comprising polyolefins on conductors, usually preheated conductors are used, this, however, being a disadvantage compared to materials, as e.g., PVC. The mechanical properties of the thin polyolefin layer are furthermore negatively affected by plastisizer migrating into it from the surrounding bedding and sheathing layers applied outside the cable core(s), which still commonly is PVC based in low voltage cables.
Furthermore, cable joints between low voltage power cables preferably are formed in such a way that, after stripping off part of the insulation layer at the end of both cables to be joined and connecting the electric conductors, a new insulation layer covering the joint conductors is often formed of a polyurethane polymer. Accordingly, it is important that the polymer composition of the original insulation layer shows a good adhesion to the polyurethane polymer used for restoring the insulation layer so that the layer is not disrupted even under mechanical stress at the cable joints.
Still further, as insulation layers of low voltage power cables usually are formed by direct extrusion onto a conductor, it is important that the polymer composition used for the insulation layer shows good extrusion behavior and, after extrusion, retains its good mechanical properties.
WO 95/17463 describes the use of a sulphonic acid as a condensation catalyst added in a masterbatch which comprises 3-30% by weight of LD, PE or EBA.
WO 00/36612 describes a Medium/High voltage (MV/HV) power cable with good electrical properties, especially long time properties. These MV/HV cables always have an inner semiconductive layer and outside that layer an insulation layer. The adhesion between these layers is always good since they are made of essentially the same material, i.e. polyethylene compounds. In contrast, the present invention is directed to a low voltage power cable and inter alia solves the problem of adhesion of the insulation layer to the conductor and problems associated with extruding directly on a conductor.
WO 02/88239 teaches how additives shall be chosen to an acid condensation catalyst.
U.S. Pat. No. 5,225,469 describes polymer compositions based on ethylene-vinyl ester and ethylene-alkyl acrylate copolymers which can be crosslinked to provide insulation coatings for wire and cable products.
EP 1 235 232 teaches that the coating layer of cables based on a composition material comprises polar groups and inorganic material.
Accordingly, it is the object of the present invention to provide a low voltage power cable with an insulation layer which shows good mechanical properties and, at the same time, shows good adhesion to polyurethane polymers and after extrusion retains its good mechanical properties. It is a further object of the invention to provide a low voltage power cable with an insulation layer having an improved resistance to deterioration of mechanical properties caused by migration of plasticisers from PVC into the layer.